Diazotroph 903303 220854477 2008-06-21T22:54:28Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. '''Diazotrophs''' are [[bacteria]] that [[Nitrogen fixation|fix]] atmospheric [[nitrogen]] gas into a more usable form such as [[ammonia]]<ref name=Postgate98>{{cite book |author=Postgate, J |year=1998 |title=Nitrogen Fixation, 3rd Edition |publisher=Cambridge University Press, Cambridge UK}}</ref>. A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. Examples of organisms that do this are [[rhizobia]] and ''[[Frankia]]'' (in symbiosis) and ''[[Azospirillum]]''. All diazotrophs contain iron-molybdenum [[nitrogenase]] systems. Two of the most studied systems are those of ''[[Klebsiella]] pneumoniae'' and ''[[Azotobacter]] vinlandii''. These systems are used because of their genetic tractability and their fast growth<ref name=DK94>{{cite journal |author=Dixon R and Kahn D |year=2004 |title=Genetic regulation of biological nitrogen fixation |journal=Nat Rev Microbiol |volume=2 |issue=8 |pages=621–31 |doi=10.1038/nrmicro954}}</ref>. ==Types of diazotrophs== Diazotrophs are scattered across bacterial taxonomic groups (mostly in the [[Eubacteria]] but also a couple of [[Archaea]]). Even within a species that can fix nitrogen there may be strains that do not fix nitrogen<ref name=Postgate98/>. Fixation is shut off when other sources of nitrogen are available, and, for many species, when oxygen is at high partial pressure. Bacteria have different ways of dealing with the debilitating effects of oxygen on nitrogenases, listed below. ===Free-living diazotrophs=== * Anaerobes—these are obligate anaerobes that cannot tolerate oxygen even if they are not fixing nitrogen. They live in habitats low in oxygen, such as soils and decaying vegetable matter. ''[[Clostridium]]'' is an example. Sulphate-reducing bacteria are important in ocean sediments (e.g. ''[[Desulfovibrio]]''), and some Archean methanogens fix nitrogen in muds and animal intestines<ref name=Postgate98/>. * Facultative anaerobes—these species can grow either with or without oxygen, but they only fix nitrogen anaerobically. Often, they respire oxygen as rapidly as it is supplied, keeping the amount of free oxygen low. Examples include ''[[Klebsiella pneumoniae]]'', ''[[Bacillus polymyxa]]'', ''[[Bacillus macerans]]'', and ''[[Escherichia intermedia]]'' <ref name=Postgate98/>. * Aerobes—these species require oxygen to grow, yet their nitrogenase is still debilitated if exposed to oxygen. ''[[Azotobacter vinelandii]]'' is the most studied of these organisms. It uses very high respiration rates, and protective compounds, to prevent oxygen damage. Many other species also reduce the oxygen levels in this way, but with lower respiration rates and lower oxygen tolerance <ref name=Postgate98/>. * Phototrophs—photosynthetic bacteria generate oxygen as a by-product of [[photosynthesis]], yet some are able to fix nitrogen as well. These are colonial bacteria that have specialized cells ([[heterocyst]]s) that lack the oxygen generating steps of photosynthesis. Examples are ''[[Anabaena cylindrica]]'' and ''[[Nostoc commune]]''. Other [[cyanobacteria]] lack heterocysts and can fix nitrogen only in low light and oxygen levels (e.g. ''[[Plectonema]]'') <ref name=Postgate98/>. ===Symbiotic diazotrophs=== * [[Rhizobia]] -- these are the species that associate with legumes, plants of the [[Fabaceae]] family. Oxygen is bound to leghemoglobin in the root nodules that house the bacterial symbionts, and supplied at a rate that will not harm the nitrogenase<ref name=Postgate98/>. * [[Frankia]]s -- much less is known about these '[[actinorhizal]]' nitrogen fixers. The bacteria also infect the roots leading to the formation of nodules. Actinorhizal nodules consist of several lobes, each lobe has a similar structure as a lateral root. Frankia is able to colonize the cortical tissue of nodules where it fixes nitrogen<ref name=Vessey />. Actinorhizal plants and Frankias also produce hemoglobins<ref name=Beckwith02>{{cite journal |author=Beckwith, J, Tjepkema, J D, Cashon, R E, Schwintzer, C R, Tisa, L S |year=2002 |title=Hemoglobin in five genetically diverse Frankia strains |journal=Can J Microbiol |volume=48 |pages=1048–1055 |doi=10.1139/w02-106}}</ref>, but their role is less well established than for rhizobia<ref name=Vessey />. Although at first it appeared that they inhabit sets of unrelated plants ([[alder]]s, [[Australian pine]]s, [[Ceanothus|California lilac]], [[bog myrtle]], [[Purshia|bitterbrush]], ''[[Dryas (plant)|Dryas]]''), revisions to the [[phylogeny]] of [[angiosperm]]s show a close relatedness of these species and the legumes<ref name=Soltis>{{cite journal |author=Soltis DE, Soltis PS, Morgan DR, Swensen SM, Mullin BC, Dowd JM, Martin PG |year=1995 |title=Chloroplast gene sequence data suggest a single origin of the predisposition for symbiotic nitrogen fixation in angiosperms |journal= Proc Natl Acad Sci USA |volume=92 |pages=2647 – 2651 |doi=10.1073/pnas.92.7.2647 |pmid=7708699}} </ref><ref name=Vessey>{{cite journal |author = Vessey JK, Pawlowski, K and Bergman B |title=Root-based N<sub>2</sub>-fixing symbioses: Legumes, actinorhizal plants, ''Parasponia'' sp and cycads |journal=Plant and soil |year=2005 |volume=274 |issue=1-2 |pages=51–78 |doi= 10.1007/s11104-005-5881-5}}</ref>. * [[Cyanobacteria]] -- there are also symbiotic cyanobacteria. Some associate with [[fungi]] as [[lichen]]s, with [[Marchantiophyta|liverwort]]s, with a [[fern]], and with a [[cycad]]<ref name=Postgate98/>. These do not form nodules (indeed most of the plants do not have roots). Heterocysts exclude the oxygen, as discussed above. The fern association is important agriculturally: the water fern ''[[Azolla]]'' harbouring ''Anabaena'' is an important green manure for [[rice]] culture<ref name=Postgate98/>. * Association with animals -- although diazotrophs have been found in many animal guts, there is usually sufficient ammonia present to suppress nitrogen fixation<ref name=Postgate98/>. [[Termite]]s on a low nitrogen diet allow for some fixation, but the contribution to the termite's nitrogen supply is negligible. [[Shipworm]]s may be the only species that derive significant benefit from their gut symbionts<ref name=Postgate98/>. ==Importance== In terms of generating nitrogen available to all organisms, the symbiotic associations greatly exceed the free-living species with the exception of cyanobacteria<ref name=Postgate98/>. ==References== {{Reflist}} ==External links== * [http://azotobacter.org Azotobacter] * [http://www.rhizobia.co.nz Rhizobia] * [http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm Frankia & Actinorhizal Plants] [[Category:Nitrogen metabolism]] [[Category:Environmental microbiology]] [[de:Diazotrophie]] [[es:Diazótrofo]] [[it:Diazotrofi]] [[sr:Азотофиксатор]] [[uk:Діазотрофи]]